The harmful effect of the ultraviolet part of solar radiation on the skin is generally known. The rays have various effects on the skin as an organ depending on their particular wavelength; so-called UV-C radiation with a wavelength below 290 nm is absorbed by the ozone layer in the earth's atmosphere and therefore has no physiological significance. By contrast, rays in the range between 290 nm and 320 nm, the so-called UV-B range, cause erythema, simple sunburn or even more or less severe burns. The narrower range around 308 nm is stated to be a maximum for the erythema activity of sunlight.
Numerous compounds are known for protecting against UV-B radiation, examples thereof being derivatives of 3-benzylidenecamphor, of 4-aminobenzoic acid, of cinnamic acid, of salicylic acid, of benzophenone, and of triazine.
It has long been incorrectly assumed that the long-wavelength UV-A radiation with a wavelength between 320 nm and 400 nm has only a negligible biological effect. However, it has now been proved by many studies that UV-A radiation is far more hazardous than UV-B radiation in relation to the initiation of photodynamic, specifically phototoxic, reactions and chronic changes in the skin. It is also possible for the harmful effect of UV-B radiation to be enhanced by UV-A radiation.
Thus, it has been proved, inter alia, that even UV-A radiation under entirely normal everyday conditions is sufficient to damage within a short time the collagen and elastin fibers which are of essential importance for the structure and firmness of the skin. This results in chronic light-induced skin changes—the skin “ages” prematurely. The clinical appearance of skin aged by light includes, for example, creases and wrinkles and an irregular, furrowed relief. In addition, the areas affected by light-induced skin aging may have irregular pigmentation. The formation of brown spots, keratoses and even carcinomas or malignant melanomas is also possible. Skin aged prematurely by everyday exposure to UV is additionally distinguished by a lower activity of the Langerhans cells and a slight chronic inflammation.
About 90% of the ultraviolet radiation which reaches the earth consists of UV-A rays. Whereas UV-B radiation varies greatly depending on a large number of factors (for example season and time of day or latitude), UV-A radiation remains relatively constant from day to day irrespective of seasonal and diurnal or geographic factors. At the same time, most of the UV-A radiation penetrates into living epidermis, while about 70% of UV-B rays are retained by the stratum corneum.
It is therefore of fundamental importance that cosmetic and dermatological light protection preparations provide adequate protection both against UV-B and against UV-A radiation.
In general, the light absorption characteristics of light protection filter substances are very well known and documented, especially since most industrialized countries have positive lists for the use of such substances, which impose very strict standards on the documentation.
Effective UV protection can also be achieved with the aid of organic or inorganic particulate UV filter substances. The protective effect toward UV rays in this case increases as the size of the particles employed decreases. The reduction in particle size simultaneously has the effect that visible light is transmitted, which is why the formulations appear transparent; an unwanted whitening effect through which the affected areas of skin become distinctly white in color now occurs only with very high particle concentrations on initial application of the cream.
In order to achieve effective protection from the sun with the aid of particulate UV filter substances, it is crucial that the primary particles are retained and the dispersion remains stable. However, the reduction in size of the particles greatly increases the surface area of the particles, so that they are prone to agglomeration owing to the increasing forces of attraction.
Agglomeration of the particles leads, however, to a decrease in the pharmaceutical stability and a reduction in the effectiveness of UV protection by the formulation, and to an increase in the scattering power in the visible region, and thus to the whitening effect. Processing of micropigments therefore requires an optimal formulation and an appropriate production process.